The recovery and utilization of natural gas and other methane-rich gas streams as an economic fuel source have required the liquefaction of the natural gas in order to provide economic transportation of the gas from the site of production to the site of use. Liquefaction of large volumes of natural gas is obviously energy intensive. In order for natural gas to be available at competitive prices, the liquefaction process must be as energy efficient as possible.
Additionally, in light of the increased costs of all forms of energy, a natural gas liquefaction process must be as efficient as practical in order to minimize the amount of fuel or energy required to perform the liquefaction.
Certain conditions, such as low cooling water temperature (below 65.degree. F.) create reductions in liquefaction efficiency in single component cycles when the compression load on the refrigeration equipment used to perform the liquefaction is not balanced with regard to the drivers or machinery utilized to run the refrigeration equipment. Compression load is the major power consuming function of a liquefaction process. A liquefaction process must be readily adaptable to varying climactic conditions, wherein the liquefaction process must be efficient at operating ambient conditions in tropical environments, as well as temperate environments and cold environments, such as the subarctic regions of the world. Such climatic conditions effect a liquefaction process predominantly in the temperature of the cooling water utilized in the production of refrigeration used to liquefy the natural gas. Sizeable variations in the temperature of available cooling water due to changing seasons or different climatic zones can cause imbalances in the various refrigeration cycles of dual cycles.
Various attempts have been made to provide efficient liquefaction processes, which are readily adaptable to varying ambient conditions. In U.S. Pat. No. 4,112,700 a liquefaction scheme for processing natural gas is set forth wherein two closed cycle refrigerant streams are utilized to liquefy natural gas. A first high level precool refrigerant cycle is utilized in multiple stages to cool the natural gas. This first high level precool refrigerant is phase separated in multiple stages wherein the effect is to return the light portions of the refrigerant for recycle, while the heavy portions of the refrigerant are retained to perform the cooling at lower temperatures. The first high level precool refrigerant is also utilized to cool the second low level refrigerant. The second low level refrigerant performs the liquefaction of the natural gas in a single stage. The drawback in this process is that the high level precool refrigerant utilizes heavier and heavier components to do lower and lower temperature cooling duty. This is contrary to the desired manner of efficient cooling. Further, the second or low level refrigerant is used in a single stage to liquefy the natural gas, rather than performing such liquefaction in multiple stages.
U.S. Pat. No. 4,274,849 discloses a process for liquefying a gas rich in methane, wherein the process utilizes two separate refrigeration cycles. Each cycle utilizes a multicomponent refrigerant. The low level refrigerant cools and liquefies the natural gas in two stages by indirect heat exchange. The high level refrigerant does not heat exchange with the natural gas to be liquefied, but cools the low level refrigerant by indirect heat exchange in an auxiliary heat exchanger. This heat exchange is performed in a single stage.
U.S. Pat. No. 4,339,253 discloses a dual refrigerant liquefaction process for natural gas, wherein a low level refrigerant cools and liquefies natural gas in two stages. This low level refrigerant is in turn cooled by a high level refrigerant in a single stage. The high level refrigerant is used to initially cool the natural gas only to a temperature to remove moisture therefrom before feeding the dry natural gas to the main liquefaction area. The use of such individual stage heat exchange between the cycles of a dual cycle refrigerant liquefaction process precludes the opportunity to provide closely matched heat exchange between the cycles by the systematic variation of the refrigerant compositions when the refrigerants constitute mixed component refrigerants.
In the literature article Paradowski, H. and Squera, O. "Liquefaction of the Associated Gases", Seventh International Conference on LNG, May 15-19, 1983, a liquefaction scheme is shown in FIG. 3 wherein two closed refrigeration cycles are used to liquefy a gas. The high level cycle depicted at the right of the flowscheme is used to cool the low level cycle as well as cooling for moisture condensation in an initial gas stream. The high level refrigerant is recompressed in multiple stages and cools the low level refrigerant in three distinct temperature and pressure stages. Alteration of the high level refrigerant composition to match the various stages of refrigeration in the heat exchanger is not contemplated.
The present invention overcomes the drawbacks of the prior art by utilizing a unique flowscheme in a liquefaction process utilizing two mixed component refrigerants in closed cycles, wherein the refrigerants are indirectly heat exchanged one with another in multiple stages including varying the refrigerant composition wherein the lighter components are available to perform the lower level refrigeration duty.